The rules of discussing class in Britain are, pleasingly, very like those of cricket. Once you know them, they seem incredibly obvious and intuitive and barely worth mentioning; if you don't know them, they are pointlessly, sadistically complicated, their exclusivity almost an exercise in snobbery in its own right. Nowhere is this more evident and yet more tacit than in relationships: people marry into their own class. It's called "assortative mating". You know this by looking around, yet there's such profound squeamishness about it that research tends to cluster around class proxies. The question goes: "Do you and your spouse share the same educational attainment?" (Translation: are you the same class?) Or: "Did you go to the same university?" (Translation: are you really, really the same class?)

This trend is immune to social progress elsewhere. If anything, people are more likely than ever to marry into their own class, as a report from the Institute for Public Policy Research showed this year. Of people born in 1958, just over a third of women had a partner from the same class as themselves: 38% married up, while 23% married down. For those born in 1970, 45% married into the same class; of those born between 1976 and 1981, 56% married into the same class, with a far smaller proportion (16%) marrying up.

Even the phrases "marrying up" and "marrying down" are sullying to use. You can't really escape the connotation that the rich are better than the poor. But I use them anyway, putting them in the grammatical equivalent of surgical gloves, because there is no right-on alternative: there's no unsnobbish way to convey a difference in class between two people. The leftwards path is to pretend class doesn't exist. Which is fine, but it's also total horse manure.

So what's it actually like, when you don't mate assortatively? Emily Wyndham married her husband 11 years ago this week. They met at Oxford University. "I'd describe my parents as working class made good," she says. "My father had to leave school at 16 for financial reasons, but he became a businessman, they built a hotel. Not anywhere nice – it was in a crap industrial coastal town they forgot to close down. In doing so, they made quite a lot of money – enough to send us to private school – so we were the first generation of our family to go to university. He's always very keenly been aware of his position in life, and always very keenly felt he was working class, and wanted to assimilate himself to become middle class. He's very class-conscious. He reads the Telegraph; he's voted Tory for years and years.

"I didn't feel aware of class differences at university. Three of my closest friends had been to comps; we were all pretty much lower middle class, all from quite similar backgrounds."

She met her future husband while she was having tea with a student from another college. "I think she actually said, 'He's very posh.' " They got together just after finals. "It was never an issue; I just knew he was very nicely spoken, as my mother would say. I think quite early on in our relationship he went off shooting. It was like he'd moved to another world that I hadn't known existed. When we were in our final term, he went to his cousin's birthday and I said, 'That sounds very posh.' And he said rather sheepishly, 'Well, he is an earl.' And I thought, 'Oh fuck. This is way outside anything I've ever experienced.' "

She met his parents one month into the relationship. "I think they were a bit underwhelmed. I smoked at the time. I'm so embarrassed to think of this now, but I think I smoked between courses and they were appalled. Generally, I got the impression that I was being looked up and down and found rather wanting. But, in my favour, his sister was going out with someone who was even more low-class than me. They wanted him to marry someone who had grown up around the corner, whose parents they knew and of whom they approved. They attached no value at all to academic prowess. They hadn't heard of my parents, so they weren't interested in me. And also, I think they just slightly thought that I was a little bit too loud – not the quietly understated, elegant person that would fit into their quietly understated, elegant lifestyle."

When Emily introduced her parents to his – at the smart Islington restaurant Granita – that went quite well. "The worst was the wedding. My parents were sending out invitations, but they were on their uppers because their business had gone to pot. So they weren't paying, but because they're quite proud, I wanted them to feel like they were still the hosts. The invitations had to come from them. And there were all these titles, and they'd been told his aged aunt would only open invitations that were correctly addressed. My mum was very much, 'They'll just have to take us as they find us.' But my dad was going, 'No, no, no, it's got to be right.' He was shitting himself that somehow he would make some terrible faux pas and our lowly beginnings would be revealed.

"In his speech, he actually said, 'It's quite clear to all of us that Tom's married above himself.' It brought the house down – in the brilliant way of the British class system, where nobody ever says anything but everybody notices everything."

Emily was 26 when she married. The wedding sounds very stressful: if you're a Catholic and not the Brideshead sort, apparently it's immediately obvious how ill bred you are to people who know about that sort of thing. I wonder why she didn't put it off a bit longer. "My biological clock had been on speed-dial for a long time. I knew at 20 that I didn't want to be with someone I couldn't foresee a future with. And Tom was not that bothered about class – he couldn't have married anybody who was a class warrior, who thought everything he stood for was awful. He had to feel that he could be himself, and he did, and so did I.

"He said, 'Marrying you has taken me out of the upper class and put me into the middle class.' I know he was joking, but equally, there's an element of truth to it."

And what about their four children, ranging in age from 18 months to 10? "I'd say they are upper middle class, or possibly even middle-middle. In purely class terms, the decision about secondary school will be major. If they go to the state school, they will very obviously be different from their grandparents and even from their parents. I don't want them to grow up feeling completely divorced from their grandparents and their cousins. Although, of course, they're already divorced from my father's side."

(Lady) Alice Douglas has been with Steve for six years. Previously, she was married to Simon, whom she met while he was serving nine years in prison for armed robbery. "If I look at all my boyfriends, mostly they'd be working class. I don't know if that was a conscious decision, but certainly as a child I was always utterly devastated if anyone found out I had a title. It was just this terrible secret. Then, when I started going out with boys, I always preferred working-class boys. Now I live in a little Welsh village that is full of farmers and everyone's pretty working class, and my favourite thing is going to the pub at weekends and hanging out with all the farmers. I like people who work the land. I guess it's a sense of history, a sense of honesty. It's just life on a more basic level."

Her first husband was a Turkish refugee, and that marriage failed because he couldn't stand living in England. "My second husband I met when I was in a drama production at a prison. He was a typical working-class young lad who had masses of intelligence – he had such a lot to offer, but had failed to do anything beyond the army. I think that, because of his working-class roots, when he went up for jobs, he didn't really believe he should get them. Probably what class gives you is a belief that you can achieve things."

They bonded over playing Macbeth and Lady Macbeth in the prison production, and married while he was still inside. He was five and a half years younger. "We had a big, crazy wedding. Most people were cool about it, and I kind of ignored the ones that weren't. I don't know that there were that many differences. There were funny things – like if I was getting some building work and people were quoting, because I'd got a title and they didn't know me very well, it always seemed it was a bigger quote than if my ex-husband asked.

"Once, we were asked to go on Richard & Judy. If Simon, my ex, ever tried to negotiate a fee for things like that, people were very dismissive and often rude to him – they'd quibble over 50 quid. Whereas if I asked, we might get £1,000. It shows how ingrained it is, that if a person is of a higher class, they're worth more."

The couple had two children, now 13 and 12, and split up over the classic things that split parents up, regardless of class: maturity, reliability, who's still in the pub and who isn't. "The children are blessed with being comfortable in any class. They are just as happy with their grandma on Simon's side on a council estate as they are with my parents."

Then she met Steve. "I fell in love with him, and for about four years, he said, 'I love you, but I can't possibly get involved. Your life is too chaotic, it's too full on, there are too many people and I want a simple life.' It took four years to wear him down."

Steve agrees with this analysis, pretty much: "It's just so manic here. We're a clash of personalities really. Alice is definitely a go-getter and I'm more laid-back. I was brought up by very working-class parents. During the 1970s, growing up, there was work for everybody. My father, a maintenance fitter, always instilled in me not to be resentful of the upper classes, or the people bred into money, because they're the ones who create the work. Alice was born into wealth and power, but she'd never use it – she's absolutely down to earth. Although I do catch her up now and then on her accent. I've looked through every dictionary I can find, and I just can't find any R that would explain her pronunciation of bath.

"When I introduced her to my parents, they probably felt more concerned for Alice, really. They thought she could have been with somebody who could have given her a lot more – somebody far more independent and financially stable. But as soon as they met her and saw how sweet she was, that was it."

Julia Stephenson met her boyfriend, Steve, seven years ago. "I'm more middle class," she says, "and he's working class. But I don't know if one can really describe one's own class. I've been pitched as more upper middle class just because of this flighty life I used to live. I didn't go to university, I left school and then went to Lucie Clayton's Secretarial College. I was a Sloane Ranger, but I think a lot of people managed to be Sloane Rangers without necessarily being very wealthy."

Her mother and father had had the same class gap, to which she attributes the marital problems that saw them divorce when she was seven. "Her parents were very snotty and disapproving – they barely spoke to him – and they sent her off to Argentina for six months, but it didn't work. And then he went off and had affairs, because I think when men feel inadequate like that, they try to get validation from elsewhere."

When Julia was young, she liked posh boys with snazzy cars, and got married in that mould at 26, but that didn't work out. "I never would have met Steve if we weren't both Buddhists. But then we went on a Buddhist course to Japan and he really pursued me, which was heavenly. He was so confident. And that's when I realised that working-class men are where it's at. If only I'd realised that before my early 40s. As it was, I was pursuing these rather effete old Etonians. I like working-class men, anyway. I like the accent, I like the fact that they're always going to pubs and also, most importantly, how handy they are. They're always doing things round the house. He's built an extension. He's got four equally handy brothers and they all came round and helped him.

"My family were just delighted that I'd met anybody at all. My dad liked him, too. It's not like I was 21 and a young virgin needing protection. There are differences that are annoying. It annoys me that he likes all his food overcooked. He doesn't want to see any blood in meat. But, no, we don't have major differences. He left school when he was 15, but I left when I was 17, anyway. He has a very inquiring mind. And he's quite cultural. I thought one of the benefits of going out with a working-class man was that I wouldn't have to go to the theatre or to the opera or ballet, but no, he loves all that. I find it quite refreshing to be with someone who's not interested in keeping up with other people, who's happy to drive around in a bashed-up old van. Having been out with upper-class men, it would be a complete nightmare to be married to one of them. All the moaning, the sexual hang-ups, the way they need to be with men all the time, and are terrified of women…"

As we talk, Steve arrives home, bearing the wherewithal to build something that will stop the dogs going downstairs in the middle of the night. "He's getting the power drill out as we speak," Julia says, ironically delighted, but still actually quite delighted.

• Some names have been changed.

When two families collide: one man's story

You don't grasp the full extent to which your family communicates in middle-class code until you bring home a partner who does not. My new girlfriend, being not remotely middle class, didn't just lack fluency in this mysterious canon; she didn't even know it existed. The only upside was she had no idea just how badly things went when she first met my parents.

I had a pretty good idea of how it would go, which is why I put it off for two years. I had left my wife to be with her, and my family was furious. How could you do this to your poor wife, they'd yell. What they really meant was, how could I do this to them? How could I swap an adorably gentle-mannered professional for a loud, semi-literate divorcee whose bookshelf held precisely four volumes, all true crime stories?

The first few minutes didn't go too badly. She had recently enrolled on a college course and, as my family makes the traditional middle-class fetish of education, this seemed like safe conversational territory. And it was, until Mum asked, "How will you cope with all the pressures of a full-time job and a college course on top of it?"

What this means is: "Please note that I'm a very caring and sympathetic person." What my girlfriend heard was, "Are you up to it?" In her world, the only way to see off such doubt is to deliver a defensively bombastic broadside about how easy she finds everything. For her, that's how to demonstrate strength and resolve. In my family's world, you demonstrate that by volunteering ambivalence. "Yes," you're meant to say, "it's a lot to take on, and there are days when I do wonder how I'm going to manage it all." What that really says, of course, is, "Please note that I'm a very thoughtful and considered person, whose modesty belies resilient self-belief." Boasting achieves the exact opposite of the desired effect. My parents exchanged troubled glances.

Each time we met, they'd ask lots of questions, eliciting more boasts. In my girlfriend's circles, questions are not the currency of good manners, but the height of rudeness: intrusive, unwelcome, vaguely intimidating. None of her friends or family ever asked me anything about myself – apart from one night, when her brother, a bit drunk, asked about my job as an A&E doctor. Then he clamped his hand over his mouth. "Sorry – you must have thought I was going on like a copper!" No, no, I reassured him, the thought had truthfully never crossed my mind.

Announcing that my girlfriend was pregnant was probably the all-time low point. "Well! What interesting children you'll have," was the best my sister could manage. We took my parents for a walk to break the news, but my girlfriend was nervous and blurted it out so clumsily that they thought she was joking. When the penny dropped, those fatal first four seconds of horrified shock, before a scramble to say the right thing, were so damning that we all pretended they'd never happened. Later on, my mum drew me to one side. "Do you want to have a baby or a family?" The implication was clear: we might be capable of breeding, but we were never going to create a family culture.

To some degree she was right. My girlfriend likes to dress our daughter in Disney-branded outfits, comfort her with a dummy and feed her sweets as snacks, all of which I surreptitiously remove from our luggage before a visit to my family.

Much of my family's disapproval is so coded that it goes over her head, but I clock every shudder and wince. Most of all, I clock the discrepancy between my family's self-image as radical, liberal, a bit bohemian, and its alarm at having to put into practice the inclusive values it so loves to extol over a glass of good wine. Anonymous